Frosted protected footing for interior stair

[sayn3ver]

This may be the wrong section.

Anyways, I'm in design phase of my shop/garage.

Initial build will be unheated until funds arrive for HVAC equipment and insulation.

My local government stipulates frost protected footings with stem walls for buildings this size (planning on a 24x40).

My plan is to keep the slab floating within the stem walls.

this means pouring the slab within the stem walls after their completion. It also means separate pours at the man door and over head door thresholds so the slab doesn't sit on those two stem walls.

This also means in addition to the frost protected strip footing at the perimeter, I will need frost protected isolated spread footings with their own individual stem walls located within the interior of the building perimeter to carry point loads of the 1st floor ceiling/loft floor at the stair opening.

I am curious then is if it's a good idea to pour frost protected footings for the mid stair landing and the bottom of the stringer for the first set of steps?

My thinking is if the goal is to allow the slab to do its own thing while a frost protected footing/stem wall is ideally remaining static, if I mount my stairs and stair landing to the slab, this creates a chance for either upward or downward slab movement and thus stair movement in relation to the rest of the structure.

if I was heating from day 1 then I probably would just do conventional thickened slab details for the point loads and no footing for the stairs or stair landing.

thoughts?

 

[Uncle murph]

This may be the wrong section.

Anyways, I'm in design phase of my shop/garage.

Initial build will be unheated until funds arrive for HVAC equipment and insulation.

My local government stipulates frost protected footings with stem walls for buildings this size (planning on a 24x40).

My plan is to keep the slab floating within the stem walls.

this means pouring the slab within the stem walls after their completion. It also means separate pours at the man door and over head door thresholds so the slab doesn't sit on those two stem walls.

This also means in addition to the frost protected strip footing at the perimeter, I will need frost protected isolated spread footings with their own individual stem walls located within the interior of the building perimeter to carry point loads of the 1st floor ceiling/loft floor at the stair opening.

I am curious then is if it's a good idea to pour frost protected footings for the mid stair landing and the bottom of the stringer for the first set of steps?

My thinking is if the goal is to allow the slab to do its own thing while a frost protected footing/stem wall is ideally remaining static, if I mount my stairs and stair landing to the slab, this creates a chance for either upward or downward slab movement and thus stair movement in relation to the rest of the structure.

if I was heating from day 1 then I probably would just do conventional thickened slab details for the point loads and no footing for the stairs or stair landing.

thoughts?

In 40yrs of building,never heard or seen that one before.You don’t say where you are but the little bit that the slab could move will not have any noticeable effect on the stairs assuming that they are made of wood,again more detail would be helpful.

 

[matt_i]

I think I'd just place some F250 foamular under the stem walls at your door openings.

Rationale is that it has a greater psf than the typical average soil when everything is distributed.

However if the rest of the subgrade starts sinking away then the Foamular is going to be under a heck of a lot more distributed load to the point where it will crush down and equalize.

I think if you do good prep work on the subgrade compaction and stone support you are not going to have a lot of issues in standard soils

Back to your staircase...I wouldn't bother and simply set it on the slab. Unless you have something structural holding up part of the building itself the loads aren't there to justify it in my opinion.

 

[ConCretin]

With all due respect, you are overthinking this. I know because I over think everything and then proceed to over engineer it for good measure.

You don't say where you are but we have pretty severe frost conditions here in Maine. I've been doing concrete for 35 years and have never seen the problems you are trying to avoid. As long as you have a decent base, you aren't going to get any noticeable movement from frost under the slab. The structure itself provides enough protection to allow the natural heat from the ground to keep the frost from being a problem.

I would insulate the inner face of the stem walls with 2" rigid insulation to minimize front penetration through the wall.

 

[sayn3ver]

I'm only located in NJ. Energy zone 4 and grow zone 6/7. I believe we are a 42" frost line.

I probably am overthinking this.

I just know that leaving it unheated could cause issues with frost heave.

There is an existing detached 20x24 garage on property already that is falling apart and failing that was built on an uninsulated slab on grade in the 1960's. The slab itself is in surprising good shape and has no major cracks or uneven settling issues. It is roughly 8" below grade on one corner which is one of the main issues besides the lack of ceiling joists which have allowed the walls to splay and ridge to crack before we purchased the property back in 2014.

the garage will be used as mostly a woodworking space on the ground level and an exercise and home office space for remote work from home space once fitted out and finished.

additionally, I was planning on installing a low or mid rise flush mount scissor jack for making vehicle maintenance easier.
I've reviewed installation guidelines from say bendpak for concrete details but don't show frost protection detailed.

I'm also unsure if it would be easier to pour the floor than cut out and do the concrete work for the scissor lift or just form and pour that first before the main slab.

I do commercial electrical work for a living and sometimes when dimensions are mission critical in relation to the finished floor elevation, cutting, removal, install then repouring that section is done especially on really large slabs where variations in finished grade occur.

 

[ConCretin]

I just know that leaving it unheated could cause issues with frost heave.

That's my point. As long as you have a decent base frost heave is rarely an issue. I suspect the garage you reference has a whole different set of problems.

With regard to the pit for your lift, it's always better to build from the bottom up rather than dig back in later. Oversize the pit base and place it first. Then form up the pit sides and haunch the slab down onto the base slab. It's cleaner, more accurate and less likely to undermine the slab this way.

 

[billconner]

Did you consider a frost protected shallow foundation, short stem walls (or slightly tall footings) and it's ready for heating when you want? Should allay all concerns about frost heave.

 

[jkuro]

Why do you want a floating slab when you have footings. Put the slab on the footings.

 

[sayn3ver]

Did you consider a frost protected shallow foundation, short stem walls (or slightly tall footings) and it's ready for heating when you want? Should allay all concerns about frost heave.

My municipality doesn't allow such for a detached building this size.

 

[sayn3ver]

Why do you want a floating slab when you have footings. Put the slab on the footings.

Because in theory compacted stone/backfill vs undisturbed soil beneath frost protected strip footings settle at different rates. Placing the slab on top of the stem walls will tend to have the perimeter of the slab remain constant with the footings while the interior slab may move due to live loading or settlement creaking cracking around the perimeter at the slab/stemwall interface. Again, I have no first hand experience or nor am I an engineer/architect, concrete contractor nor builder.

 

[billconner]

My municipality doesn't allow such for a detached building this size.

Then llwillys solution works. It effectively is a frost protected shallow foundation for the slab, solving your concerns and being ready for heating.

 

[billconner]

Because in theory compacted stone/backfill vs undisturbed soil beneath frost protected strip footings settle at different rates. Placing the slab on top of the stem walls will tend to have the perimeter of the slab remain constant with the footings while the interior slab may move due to live loading or settlement creaking cracking around the perimeter at the slab/stemwall interface. Again, I have no first hand experience or nor am I an engineer/architect, concrete contractor nor builder.

You are correct IMHO. I would specifically not want slab tied to stem walls.

 

[jkuro]

Here is an example but there are many other ways to do it.

 

[sayn3ver]

Here is an example but there are many other ways to do it.

I'm not a fan of that detail myself.

1) we have a lot of termite activity in town. The suburb I live in was a post wwii boom town. Lots of cookie cutter 800-1000sqft ranchers and bungalows. Most on crawl space.

I have a relative who is a contractor in town and he hasn't been into a home in town that doesn't have at least some mud sill, rim board and joist damage from termites if not more.

my own home has had the areas under the front and back door (concrete steps on the exterior with no proper flashing or termite barrier behind them) replaced prior to purchasing by the previous owner cause that's where they typically enter in these homes. Bad step detail.

We had some active termite activity outside the home prior to purchase. A perimeter and crawl space termidor treatment and me physically removing all wood and bushes around the foundation, paying to have the grade corrected and reestablished and installing longer undergrounddrain leaders seem to have resolved them for the time being.

That detail has to me a high risk termite path even with a metal termite shield installed as there is basically no distance for them to have to climb vertically to get back into the wall cavity and expose themselves.

Secondly, that still leaves the slab resting on the stem stem wall which many argue less desirable than having the slab float inside the stemwalls to allow vertical slab movement.


after more consideration, I'll probably just run a strip footing and 6" stemwall for the three internal column footings I was concerned with. This will continue the elevated masonry wall in the garage space allowing safer mopping, water spills, snow melt from a vehicle, etc. I'll just have to frame a 2x6 wall there and lose a few inches of floor space.

the last post/column was located in an exterior wall anyhow and will just incorporated the larger 24x24 footing pad into the strip footing at that location.

upon others recommendations I will let the stair framing land right on the slab. I agree I doubt there will be enough slab movement to cause real issue with wooden stairs.

As for the door entries I think keep my slab pour inside the exterior walls and I will pour those threshold areas separately. This will allow me to create a small curb for the overhead door to land on to keep water out.

the preman door I could do the same or just shim that door up to give it clearance for a nice commercial style carpet or rubber entry floor mat.

they can easily be connected to the stemwall by leaving rod exposed during the stemwall block laying.

it also allows the internal slab and driveway slab to meet but remain separate for movement. I realize the apron slab need to have a turned down reinforced footing detail similar to the frost protected shallow foundation.